This invention relates to a liquid propellant gun of the kind in which liquid propellant is burned in a combustion chamber to fire a projectile from the gun. It relates particularly to a cam operated, externally driven, liquid propellant gun having a slim profile so that a plurality of single barrel gun modules can be conveniently clustered in a variety of configurations. The present invention also relates particularly to an internal water cooling arrangement which injects a small quantity of water into the combustion chamber for cooling by internal vaporization after the firing of each round and which also serves to fill the combustion chamber with water and to purge propellant from the combustion chamber in the event of a misfire.
The present invention has particular utility for high performance, high rate of fire guns in the 20 to 35mm size. The present invention is not, however, limited to guns of this size.
The existing weapons used by the armed services use solid propellant cartridges. These existing weapons carry the solid propellant in cases, and the cases form a substantial part of the overall weight and overall size of the cartridge. This in itself imposes serious drawbacks and limitations on the installation and use of such weapons, because the projectile feed mechanism and related storage facilities must be large enough and strong enough to store and transport not only the projectile itself but also the related solid propellant and case.
Solid propellants have a further inherent disadvantage because of the fact that solid propellants characteristically develop a high peak temperature. In many gun installations it is necessary to fire long bursts in multiple engagements. Such projected firing schedules produce severe thermal loads on the gun and often cause barrel errosion with the existing solid propellant weapons.
Automatic guns used in antiaircraft roles are a good example of guns subjected to severe firing schedules. Long bursts are needed to achieve high cumulative kill probabilities. These gun systems must also engage multiple targets in rapid succession with little or no time between burst for adequate cooling. A severe barrel cooling problem results which is a primary factor in limiting system effectiveness. The reduced accuracy associated with premature barrel erosion can effectively destroy gun capability during a single engagement. The alternative is to increase the number of available mounts to achieve an acceptable firing schedule. This results in additional weight, complexity, cost and maintenance problems, and is therefore an unacceptable solution.
The problem has long been recognized in high performance, gun installations such as the U.S. Navy 40 mm Bofors automatic gun and the Oto Melara 76/62. In both cases a classic approach to barrel cooling has been taken, i.e. water jacketing of the exterior barrel surface. However, even with exterior water jacketing, the heat transfer rate may be too limited for some applications.
The problems of severe thermal loads and barrel erosion also occur in drilling by cannon excavation. In cannon excavation the firing rate is relatively low but the duty cycle is sustained for long periods of time, and this produces severe thermal loads on the barrel.
It is one important object of the present invention to provide a more effective means for barrel cooling. This object is achieved in the present invention by internal water cooling. The way in which the internal water cooling is incorporated in a liquid propellant gun of the present invention also permits the mechanism for injecting the water for cooling to be used as a water purge system for purging the combustion chamber of liquid propellant in the event of a misfire, and this system and mode of operation constitutes another, specific object of the present invention. The internal water cooling system will be reviewed in more detail below in the Summary of the Invention and in the Detailed Description of the Preferred Embodiments of the present invention. At this point the applicants would like to point out that, because the water does impinge directly on the heated gun bore surfaces in the present invention, high heat transfer rates are realized and the effectiveness of the internal water cooling permits significant increase in burst length and frequency in automatic guns. It also permits a significant increase in length of the duty cycle in such applications as drilling by cannon excavation.
There are a number of recognized technical objectives for high performance guns. In general, these include: (1) increased velocity and rate of fire; (2) lower gun and ammunition weight; (3) improved interior and exterior ballistic performance; (4) decreased erosion, flash and smoke; (5) reduced recoil loads; (6) elimination of cases, links and sabots; (7) improved reliability and safety; and (8) versatility--application to a wide range or requirements.
In addition to these general improvements, the following characteristics are recognized as being factors lacking in the prior art and needed to enhance the applicability of future gun systems as compared to the prior art: (1) a gun of minimum cross section to assure maximum versatility of installation on shipboard, vehicle and aircraft mounts; (2) an envelope that will assure retrofit capability or single or multibarrel high performance 30 or 35 mm liquid propellant guns in existing 20 mm installations; (3) a mechanism design capable of employing high density, low drag projectiles currently in the inventory or in an advanced stage of development; (4) at the 30/35 mm scale--utilization of existing projectile designs (with only minor modifications) to eliminate immediate requirements for development of new projectiles, and muzzle velocities in excess of 4000 ft. per second employing high sectional density projectiles to provide adequate standoff, short time of flight, and high projectile payload; (5) a gun mechanism construction adaptable to operation at higher muzzle velocities when adequate projectiles are available; (6) stationary barrel construction with rotating cam feed mechanism to provide significant reduction in gun drive power requirements and quicker acceleration to full firing rate; (7) simplified gun harmonization at all firing rates by elimination of tangential projectile velocity components associated with rotating barrel systems.
A further requirement which has been placed on gun development in guns of this size range is that the gun must be applicable across the board to sea, air and ground needs for the three services. These include (but are not limited to) small craft point defense, landing craft armament, retrofit of existing fixed wing aircraft and antiaircraft and antivehicle ground applications where rate of fire and configuration constraints vary widely. Some missions require single barrel guns with relatively low, adjustable rates of fire (0 to 1000 rpm). Others involve multibarrel installations at intermediate rates of fire (2000 to 3000 rpm), and finally there are those which require very high rates of fire (4000 to 6000 rpm). It can be seen that this range of rate of fire indicates that automatic guns are needed from one to eight barrels.
Liquid propellant guns have a characteristic low peak temperature. Because a liquid propellant will ignite in the bulk mode, it can be ignited, as by an electrical spark device immersed in the liquid propellant, without the need to vaporize the propellant prior to ignition. Liquid propellants are high energy density liquids and can be burned in discrete pulses to produce high combustion pressures. Pulsed burning of a liquid propellant can produce combustion pressures in the range of 10,000 to 80,000 psi and even higher. The magnitude of the average combustion pressure in such pulsed burning can be controlled by the amount of expansion permitted. Higher average combustion pressures can be produced by permitting less expansion.
The liquid propellant gun can produce a flatter combustion chamber pressure-time characteristic than a solid propellant gun. Hence, performance equivalent to a solid propellant gun can be obtained at lower pressure. High cyclic rates of fire are possible with a liquid propellant gun. Because the propellant is a liquid, the propellant can be easily pumped to the firing chamber from a storage area remote from the gun itself. This permits flexibility of installation. Because the cartridge feeding system of the liquid propellant gun carries only the projectile itself, the projectile feed system can be simplified and can be made considerably lighter in weight than for a conventional gun. Or, a considerably larger projectile size and weight can be used for higher performance without having to increase the size of the projectile feed mechanism. This is especially important in permitting larger bore liquid propellant guns to be incorporated in retrofit installations as replacements for existing smaller bore solid propellant guns.
Liquid propellant guns also permit slim profiles which provide desirable configuration versatility. Because the liquid propellant gun permits a low profile, clean exterior design, an individual liquid propellant gun module or a modular grouping of liquid propellant gun modules can be installed in locations that would not accomodate a conventional gun.
It is another important object of the present invention to incorporate the inherent advantages of a liquid propellant gun in a modular gun of the kind incorporating a drive cam and a control cam.